NEW YORK, DEC. 21 -- In an effort to win concessions from Pan Am Corp.'s unions, Braniff Inc. is enlarging its takeover bid for Pan American World Airways to include the carrier's Northeast shuttle service and its commuter airline.

Pan Am has given Dallas-based Braniff until Tuesday to submit a plan for obtaining $200 million a year in wage concessions for four years from Pan Am's unions, according to sources in the two companies. The unions, in turn, are looking for assurances that Braniff would keep Pan Am intact.

Pan Am shares fell 50 cents to close at $3 today on the New York Stock Exchange.

The coalition of four Pan Am unions negotiating with Braniff Chairman Jay Pritzker includes the Teamsters and unions representing flight attendants, pilots, flight engineers and mechanics. A fifth union representing transport workers is not included in the talks.

The labor group has been trying to find an investor or buyer for Pan American World Airways for about a year.

The Pan Am Shuttle -- which operates among New York, Boston and Washington in competition with Eastern Airlines -- is "dear to the hearts" of Pan Am's unions, said a source close to the negotiations who requested anonymity. "Severing the shuttle relationship would be difficult and not necessarily productive."

Both the shuttle service and the commuter airline, Pan Am Express, are widely viewed as fairly profitable operations within an otherwise bleak financial landscape for the once-mighty airline company. Pan American World Airways operates mainly between the United States and foreign cities and suffers from a lack of more lucrative domestic routes.

The main airline subsidiary posted a net loss of $29.3 million for the first nine months of this year, compared with a loss of $283.6 million in the same period in 1986. Its revenue edged up to $2.2 billion from $1.94 billion.

Pritzker's original plan called for the main airline to be spun off from the parent company and merged with Braniff. Pan Am shareholders would have received stock in a new company and nonvoting shares in Braniff, while Pan Am employes would have gotten a 20 percent stake in the new company in exchange for the contract concessions.

The plan was recently approved by Pan Am's board.

Under the enlarged bid, Braniff would probably take a smaller proportion of the stock in the new company to pay for the shuttle and the commuter line, airline analysts suggested. "I'm sure there is merit in keeping all of those {Pan Am} pieces together," said Robert Joedicke, an analyst for Shearson Lehman Brothers Inc., calling the shuttle operation "a Pan Am identity" that is an integral part of the company's network.